Light absorbed by the photosynthetic pigments induces charge separation in photosystems I (PSI) driving electron transport from water to NADP+. One of the early electron acceptors in photosystem I (PSI) is a very low potential iron-sulfur cluster, Fx. Understanding the mechanism of action of PSI requires a knowledge of the structure of Fx and the structural changes induced by the electron transfer process through Fx. Although many of the components of PSI and their function are known, there is a little information about their special arrangement and their interaction with the amino acid residues of the polypeptide chains. Site directed mutations were performed on the psaB gene from the cyanobacterium on the two conserved cysteines 556 and 565. Each cysteine was substituted once with serine and once with histidine resulting in overall of four mutants of the B subunit, these cysteines are proposed to serve as two of the four ligands that binds the iron in Fx. We present the X-ray absorption fine structure (XAFS) spectra of the mutated Fx in comparison to the native Fx spectra. The XAFS analysis of the mutated metal site provides a direct assignment of the different ligands.